Electrode and discharge operation depending on polarity

This picture shows the aspect of the plasma at and near the electrode in a clear discharge tube filled with neon and mercury. This lamp is a clear version of Philips's TLD 16W HF, a fluorescent tube optimized for an operation on HF drivers. Here, the lamp is operated from a standard 50 Hz circuit and pictures are taken with a shutter speed faster than 10 ms in order to capture the aspect of the discharge during a specific polarity. The top image shows the electrode operating in the cathode mode, resulting in some neon emission (in red) due to highly energetic electrons in the negative glow. We can also see the Faraday dark space between the negative glow (left) and the positive column (right). Current flows in that zone but electrons are not energetic enough to cause the excitation and ionization of the lamp's atmosphere. The bottom picture shows the electrode operating in the anode regime. There, the gas excitation is more homogeneous and electrons are energetic enough to excite mercury vapor only.